Developments of miniaturized and high-speed semiconductor devices (IC (Integrated circuit) chips) used in a server, a network device, etc. have been progressing year by year. Further, along with the developments of miniaturized and high-speed IC chips, a rate of signals inputted and outputted into/from the IC chips has also been increased, so that the signal rate in the next generation IC chips is expected to be 25 Gbit/sec per channel.
Along with the increases of a signal rate described above, a large transmission loss is generated even in transmission at a relatively short distance (about 30 cm), thereby deteriorating signals. For example, a large transmission loss is generated between an IC chip and another IC chip mounted on the same substrate, or between an IC chip and a transmission module such as an optical transceiver, thereby deteriorating the signals. In the wiring on a printed circuit board, a transmission loss of 0.8 dB/cm is usually generated, and a transmission loss of about 0.4 dB/cm is generated even in the wiring on an expensive substrate formed of a resin for high-rate signals. In order to reduce the transmission loss and to suppress the signal deterioration, there is a method of using a special substrate (for example, ceramic substrate) formed of more expensive resins, but the effect thereof is limited.
Further, when a substrate on which an IC chip is mounted is mounted on another substrate, signals are deteriorated caused by the transmission from the substrate on which an IC chip is mounted to another substrate. In the following description, a substrate on which an IC chip is mounted will be called as “package substrate”, and an IC chip and a package substrate will be collectively called as “IC package” in some cases. Moreover, a substrate on which an IC package is mounted will be called as “motherboard” to be distinguished from the package substrate in some cases. However, the naming and the distinction described above are used only for the sake of convenience in the explanation.
Further, a build-up substrate including a core layer therein is usually used in the package substrate, and a signal transmission between the IC package and the motherboard is performed through a through-hole provided in the package substrate (build-up board). However, the through-hole often causes signal deteriorations. Moreover, since a size (diameter) of the through-hole cannot be reduced in a core layer, it is difficult, while maintaining wiring density, to maintain good transmission property.
Further, a high-performance and large-scale IC includes a large number (several thousands) of connection pins. Then, the IC package including the large-scale IC is often connected to the motherboard through two-dimensionally arrayed connectors (sockets) called as an LGA (Land Grid Array). However, it is not easy to let high-rate signals (25 Gbit/sec) pass through the LGA. Moreover, when the IC package and the motherboard are connected to each other through the two-dimensionally arrayed LGA, a through-hole connection is inevitable so as to output signals from a land on the motherboard to another IC or another module. However, it has already been described above that the through-hole contributes to signal deteriorations.
For the reasons described above, the signals are greatly deteriorated even in transmission at a short distance (about 30 cm), and an expensive compensation circuit having large power consumption is required for compensating such deterioration.
Therefore, a technique of mounting a transmission module on the package substrate on which an IC chip is mounted has been proposed in Japanese Patent Application Laid-Open Publication No. 2000-269545 (Patent Document 1). When the transmission module is mounted on the package substrate, a distance between the IC chip and the transmission module is shorter. That is, a signal transmission distance is shorter. Moreover, the signals can be transmitted between the IC chip and the transmission module without through a core layer.